In the turbine section of a turbine engine, the turbine rotor is circumscribed by a shroud such that the shroud is adjacent to the tips of the rotor blades extending from the hub of the rotor. The shroud serves to channel the combustion gases through the turbine section of the turbine engine and prevents the bulk of the turbine engine's combustion gases from bypassing the turbine rotor blades. However, a portion of the gases are able to bypass the rotor blades through a gap present between the rotor blade tips and the shroud. Because the energy of the gases directed through the rotor blades is used to rotate the turbine rotor assembly and any compressor upstream of the turbine section, turbine engine efficiency can be increased by limiting the gases which are able to bypass the rotor blades through this gap.
Manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Any rubbing contact between the rotor blade tips and the shroud will spall the tips of the rotors. Spalling will tend to further increase the gap described above, thereby reducing engine efficiency. In addition, spalling tends to promote structural fatigue in the rotor blades, causing the useful life of the rotor to be shortened.
As an alternative, it is well known in the art to form a dynamic seal between the rotor blades and the shroud by forming an abrasive tip cap on the end of one or more rotor blades, and more preferably, on each rotor blade. During operation of the turbine, the abrasive tip caps abrade a groove in the shroud as a result of numerous "rub encounters" between the abrasive tip caps and the shroud. The groove, in cooperation with the rotor blade tips as they partially extend into the groove, forms a virtual seal between the rotor blade tips and the shroud. The seal reduces the amount of gases which can bypass the rotor blades, and thereby improves the efficiency of the turbine engine.
Various materials and processes have been suggested to provide a suitable abrasive tip cap on turbine rotor blades. Typical abrasive materials used include silicon carbide, aluminum oxide, tantalum carbide and cubic boron nitride. Aluminum oxide, or alumina, is generally preferred because of its high temperature capabilities and oxidation resistance. In that such abrasive particles do not provide a structurally sound material, they are incorporated with a metal matrix, including for example, nickel or cobalt-base alloys, to provide a sufficiently strong structure which can be bonded to the blade tip. However, the thickness of such a metal matrix is often limited because of the structural weakness of the abrasive composition.
In some applications, it is conventional to apply the abrasive composition to the rotor blade tip using a thermal spray technique, such as plasma spraying or detonation gun spraying. While suitable for many purposes, thermal spray techniques are inefficient in that only part of the abrasive composition contacts and adheres to the rotor blade tip, while much of the thermal spray completely misses the target. More importantly, thermal spraying damages or destroys the morphology of the abrasive particles, making them unsuitable for the intended purpose. In addition, subsequent processes are typically necessary to provide the adhesion and structural integrity necessary for the abrasive composition to survive the hostile environment of a turbine engine. Such steps often include adhering the abrasive composition to the blade tip during a first heating and cooling cycle, and later depositing an additional quantity of the metal matrix over the abrasive composition through a second heating and cooling cycle, such as during hot isostatic pressing. As an alternative, it has also been suggested to melt the tip of the blade, such as with lasers, introduce the abrasive to the blade tip, and then resolidify the blade tip.
While the above processes may be suitable for some turbine blade structures, turbine blades used in modern gas turbine engines are often fabricated from cast high temperature nickel-base superalloys having a single crystal microstructure. Single crystal blades are characterized by extremely high oxidation resistance and mechanical strength at elevated temperatures, which are necessary for the performance requirements of modern turbine engines. However, the single crystal microstructure must not be affected by the process by which the rotor blade abrasive tip caps are secured to the rotor blades. In particular, the process must not recrystallize the microstructure such that the high temperature properties of the rotor blade are lost or diminished. As a result, processes which entail melting the rotor blade tip are entirely unacceptable, and repeated thermal cycling of the rotor blades runs the risk of degrading the single crystal microstructure.
U.S. patent application Ser. No. 07/941,618, now U.S. Pat. No. 5,264,011, to Brown et al. and assigned to the assignee of this patent application, teaches a method by which a rotor blade abrasive tip cap can be bonded to a single crystal rotor blade in which degradation of the microstructure of a single crystal turbine rotor blade is minimized. The method entails the use of an abrasive preform whose composition includes a metal powder matrix containing a cobalt-base braze alloy and a cobalt alloy containing boron.
The abrasive preform is semi-rigid and thick, permitting it to be physically placed directly on the rotor blade tip. Therefore, the need for thermal spray operations to deposit the abrasive composition onto the rotor blade tip is eliminated. Another advantage of the thick configuration of the preform is that the resulting abrasive blade tip cap has sufficient thickness so as to provide stock for machining to tolerance while also retaining adequate thickness to perform repeated rub encounters with a turbine engine shroud over the life of the turbine engine.
The abrasive composition is formulated to take advantage of the high temperature capabilities of a single crystal rotor blade such that a single heating cycle can be used to both consolidate the abrasive composition and bond the abrasive preform to the tip of the rotor blade. As a result, the abrasive composition and method taught provide an efficient and economical process for forming abrasive tip caps on single crystal rotor blades.
Though the above process and abrasive composition have proven to be quite satisfactory for many applications, it has been discovered that some degradation of long-term mechanical properties occurs in the single crystal turbine rotor blades. Moreover, multiple processing cycles further degrade the mechanical properties of the blades, practically eliminating the ability to both rework and retip the single crystal rotor blades processed in this manner. Further, the above process may degrade the microstructure and properties of an equiaxed grain turbine blade so severely as to render the blade unusable.
Thus, it would be desirable to provide an abrasive composition which can be readily formed into a rigid abrasive blade tip cap preform, to permit the preform to be physically placed on a rotor blade tip prior to bonding, and provide a bonding cycle which prevents degradation of the microstructure of a cast turbine rotor blade, whether it is equiaxed grain or single crystal.